Octatetraenes structure

The structure of product 3 is tub shaped, in agreement with the corresponding dibenzocyclo-octatetraene. 

The most stable conformation of both hexatriene and octatetraene is the al -s-trans one. Figure 2 represents these structures schematically. 

Vibrational frequencies of hexatriene and octatetraene have been reported by several authors21,24-26,36. The increase in the size of these molecules with respect to butadiene limits the use of highly accurate levels of calculation, so that a good choice of scaling factors is necessary to obtain useful results. Kofraneck and coworkers21 have shown that employing scale factors determined from vibrational data for trans structures alone does not give a balanced description of cis and trans structures. 

FIGURE 31. Spectrum obtained after ionizaton of all-trans octatetraene in argon. Note the occurrence of different rotamers (labelled o to t). The fine structure in the first absorption band of the t rotamer is due to site splittings279... 

Cyclo-octatetraene reacts with iron carbonyls to form complexes with the compositions [Fe(CO)3(C8H8)], [Fe2(CO)6(C8H8)], and [Fe2(CO)7(C8H8)] 152, 168, 180). Nakamura and Hagihara 166) report that the complex [Fe(CO)3(C8H8)] decolorizes bromine in carbon tetrachloride and shows absorption bands in its infrared spectrum at 699, 716, and 720 cm-1 due to cis-double bonds. They suggest structure (XVI) for this complex, i.e., the hydrocarbon retains its tub form in the complex. These results are con-... 

A single crystal X-ray structural determination of the binuclear complex [Fe2(CO)6(C8H8)] (64a) shows it to have the structure (XVIII) and not one of the previously suggested structures 22, 54,152,166,174) including tub and planar forms of the cyclo-octatetraene ring. The cyclo-octatetraene ring in the complex approximates to a chair form in which the four carbon atoms associated with each iron atom are planar or very nearly so. The observed Fe—C and C—C distances in this complex are compared... 

The transformation of cyclooctatetraene 315 to 316 has been achieved and is quoted387" as an example of a [ a + 7i2a] cycloaddition, and the alternative transposition 317 - 318 is also known.3876 For polyazacyclo-octatetraenes (319, 321, 323, 324), this type of cyclization can lead to heteroethylenic structures 320 or aromatic systems (322, 262, 15). 

No examples of 321 - 322 are known, though the aromatic character of 322 should favor such a transformation. Simmons et al.339 3g8 have examined the related isomerization 324 - 15 and concluded that the polyazapentalene forms 254b, 255, 328, and 329 are stable. Related systems 253,3338 254a,335 and 263350 have been prepared, and in all cases the azapentalene structure has been unambiguously confirmed, in two cases (255389 and 263351 390) by X-ray structures. Tetraazacyclo-octatetraenes 324 have not been detected and they are assumed to be transformed rapidly into azapentalenes. 

The reaction of the octatetraene is faster, because the easily achieved spiral transition structure corresponds to the allowed [K8a] conrotatory process ... 

Electro-optical modulators are other examples whose efficiency is enhanced in the presence of ion radicals. These devices are based on sandwich-type electrode structures containing organic layers as the electron-hole-injecting layers at the interface between the electrode and the emitter layer. The presence of ion radicals lowers the barrier heights for the electron or hole injection. Anion radicals (e.g., anion radicals from 4,7-diphenyl-l,10-phenanthroline—Kido Matsumoto 1998 from tetra-arylethynyl-cyclo-octatetraenes— Lu and co-workers 2000) or cation radicals (e.g., cation radicals from a-sexithienyl—Ku-rataetal. 1998 1,1 -diphenyl-2-[phenyl-4-Ar,ALdi(4 -methylphenyl] ethylene—Umedaand co-workers 1990, 2000), all of them used as electron or hole carriers. 

Figure 19 Structures of S, /S0 conical intersections in conjugated hydrocarbons showing the —(CH)3— kink structure in all-trans-octatetraene, benzene, and cyclohexadiene. Also the typical triangular Dj/D0 conical intersection for H3 is illustrated.

On the other hand, hexatriene must take a helical structure.6 By analogy with the octatetraene case, it may be inferred that electronic effects will have a smaller influence and steric effects a larger influence than in the cyclobutene torquoselectivity. This is indeed what was found by Houk and co-workers.7... 

The theoretical models start with Kekule s [44] description of benzene, as having two structures. Later Hiickel [45,46] discovered his [4 +2] and [4n] rules, and was able to account for the stability of benzene ([4 +2]) and the instability of cyclobutadiene and cyclo-octatetraene (both [4 ]). The [4 +2] compounds were called aromatic after benzene, while the [4n compounds were given the designation anti-aromatic. 

Fig, 4. Transition structure for the electrocyclic ring closure of 1,3,5,7-octatetraene to 1,3,5-cyclooc-tatriene... 

There are abundant examples in the literature which reveal that the thermal electrocyclization of 8e systems to form eight-membered rings proceeds with lower activation energies than for the lower vinyl-og, (Z)-hexatrienes. The helical geometry of the transition state leads to less steric congestion about Ae reacting termini of the octatetraene and this likely accounts for the facility of the thermal 8e conrotatory process. Minor structural differences can induce cycloreversion of the process. ... 

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